Polarizing films which are now being used most commonly comprise a base film formed of a polyvinyl alcohol resin and endowed with polarizing power by an iodine compound and/or a dichroic substance such as an acid dye or direct dye having a preselected structure. In this type of polarizing films, durability is usually secured by covering both sides thereof with filmy materials (hereinafter referred to as protective coat layers) having moisture resistance and, on at least one side, transparency. That is, the shortcoming of the inner polarizing film layer (hereinafter referred to as the polarizer layer) significantly lacking in durability from its nature has been overcome by protecting both sides thereof with the protective coat layers to secure adequate durability for practical purposes.
As an important constituent element of liquid-crystal display devices, polarizing films are now being used in large quantities. However, as the field of application of liquid-crystal display devices is enlarged, there is an increasingly strong demand for improved durability, particularly moisture resistance and thermal resistance, of the polarizing films used therein.
A number of propositions have been made to meet such a demand. To sum up, these propositions can be classified into three methods. The first method is to use a conventional polarizer comprising a combination of a polyvinyl alcohol resin and a water-soluble dichroic dye and protect it with protective coat layers formed of a material (such as a cellulose acetate resin, an acrylic resin, a polyester resin, a polyurethane resin or the like) having better durability than the polyvinyl alcohol resin. This method can produce a considerable improvement in durability. However, such an improvement in durability has its limit because the edges of the polarizer exposed at the cut ends of the polarizing film have poor moisture resistance and the base resin of the polarizer has inherently low thermal resistance. The second method is to form a polarizing film of a hydrophobic polymer having a polyene structure with conjugated double bonds. However, this method has not yet attained technological completeness because, though having improved moisture resisntance, such a polarizing film has such disadvantages as a change in transmittance due to an augmentation of polyene structure induced by heat or other causes, a basically low degree of polarization, and the like. The third method is an attempt to form a polarizing film or polarizer by coloring a hydrophobic polymer, typified by polyesters, polyamides and the like, with a dichroic colorant and then stretching the resulting film, and the present invention basically belongs to this method. According to the third method, it is possible in principle to radically solve the long-standing problems of moisture resistance, thermal resistance and the like. In practice, however, few dichroic colorants exhibiting a high degree of dichroism in such a hydrophobic polymer have been proposed and this fact imposes restrictions on the completion of the technique for forming polarizing films according to the third method.
The prior art relating to durable polarizing films include, for example, Japanese Patent Laid-Open No. b 84409/'82, Japanese Patent Laid-Open No. 68008/'83 in the name of the present inventors, and the like. All of them are concerned with polarizing films to which dichroic dyes developed for use with liquid crystals are applied. Immediately after manufacture, these polarizing films may have polarizing performance comparable to that of conventional PVA type polarizing films. However, they are disadvantageous from a practical point of view in that their long-term use, especially in a heated state, causes a marked reduction in polarizing power. The main cause for this disadvantage lies in the fact that dichroic dyes for use with liquid crystals have generally been selected so as to have a structure which permits them to be dissolved in liquid crystals at the highest possible concentration. However, in a film base material comprising a hydrophobic resin such as polyethylene terephthalate, the molecules of such a dye are thought to shift easily owing to their thermal motion or the like, especially in a heated state, and disturb their own orientation. Moreover, during the manufacture of such a polarizing film, the film having passed through a stretching step is usually subjected to a heat-treatment step for the purpose of preventing its shrinkage or the like and securing its dimensional stability. Where a colorant (such as a dichroic dye for use with liquid crystals) having very high solubility in the hydrophobic base resin is used, the degree of polarization of the resulting film may be considerably high immediately after the stretching step. However, this film has the advantage that, after having passed through the heat-treatment step, its degree of polarization is substantially reduced. Accordingly, if it is desired to achieve a high degree of polarization in such a polarizing film, the stretched film cannot be subjected to an adequate heat treatment for heat-setting purposes, making it possible to obtain a polarizing film having a sufficient degree of dimensional stability and a stabilized degree of polarization. On the contrary, if it is desired to obtain a polarizing film having a sufficient degree of dimensional stability and a stabilized degree of polarization, the stretched film must be subjected to an adequate heat treatment, making it impossible achieve a high degree of polarization.
In order to solve these problems, the present inventors have made extensive investigations and have found that, in polarizing films using a hydrophobic resin as the film base material, the most important characteristic required of a colorant used to solve the above-described problems is a high degree of dichroism and, moreover, such a colorant must have a certain degree of pigment nature. The present invention has been completed on the basis of these discoveries.